1. Field of the Invention
The present invention relates to a compression ignition type engine.
2. Description of the Related Art
In the past, in an internal combustion engine, for example, a diesel engine, the production of NOx has been suppressed by connecting the engine exhaust passage and the engine intake passage by an exhaust gas recirculation (EGR) passage so as to cause the exhaust gas, that is, the EGR gas, to recirculate in the engine intake passage through the EGR passage. In this case, the EGR gas has a relatively high specific heat and therefore can absorb a large amount of heat, so the larger the amount of EGR gas, that is, the higher the EGR rate (amount of EGR gas/(amount of EGR gas + amount of intake air), the lower the combustion temperature in the engine intake passage. When the combustion temperature falls, the amount of NOx produced falls and therefore the higher the EGR rate, the lower the amount of NOx produced.
In this way, in the past, the higher the EGR rate, the lower the amount of NOx produced can become. If the EGR rate is increased, however, the amount of soot produced, that is, the smoke, starts to sharply rise when the EGR rate passes a certain limit. In this point, in the past, it was believed that if the EGR rate was increased, the smoke would increase without limit. Therefore, it was believed that the EGR rate at which smoke starts to rise sharply was the maximum allowable limit of the EGR rate.
Therefore, in the past, the EGR rate was set within a range not exceeding the maximum allowable limit. The maximum allowable limit of the EGR rate differed considerably according to the type of the engine and the fuel, but was from 30 percent to 50 percent or so. Accordingly, in conventional diesel engines, the EGR rate was suppressed to 30 percent to 50 percent at a maximum.
Since it was believed in the past that there was a maximum allowable limit to the EGR rate, in the past the EGR rate had been set so that the amount of NOx produced would become as small as possible within a range not exceeding that maximum allowable limit. Even if the EGR rate is set in this way so that the amount of NOx produced becomes as small as possible, however, there are limits to the reduction of the amount of production of NOx. In practice, therefore, a considerable amount of NOx continues being produced.
Therefore, known in the art is a diesel engine which disposes in the engine exhaust passage a NOx absorbent which absorbs NOx when the air-fuel ratio of exhaust gas flowing into the NOx absorbent is lean and releases and reduces the absorbed NOx when the air-fuel ratio of the exhaust gas flowing into the NOx absorbent is rich or the stoichiometric air-fuel ratio (International Publication WO93/07363). In this diesel engine, in the same way as the conventional diesel engines, the air-fuel ratio in the combustion chamber is lean, therefore the NOx produced in the combustion chamber is absorbed by the NOx absorbent. Further, in this diesel engine, when the amount of the NOx absorbed in the NOx absorbent exceeds a predetermined amount, the air-fuel ratio in the combustion chamber is temporarily made rich and therefore the NOx is released from the NOx absorbent and the released NOx is reduced.
In such a diesel engine, however, there is the problem that when the air-fuel ratio in the combustion chamber is made rich, a large amount of smoke is produced.
On the other hand, the present inventors discovered in the process of studies on the combustion in diesel engines that if the EGR rate is made larger than the maximum allowable limit, the smoke sharply increases as explained above, but there is a peak to the amount of the smoke produced and once this peak is passed, if the EGR rate is made further larger, the smoke starts to sharply decrease and that if the EGR rate is made at least 70 percent during engine idling or if the EGR gas is force cooled and the EGR rate is made at least 55 percent or so, the smoke will almost completely disappear, that is, almost no soot will be produced, even if the air-fuel ratio in the combustion chamber is made rich. That is, the present inventors discovered a new combustion method enabling the air-fuel ratio in the combustion chamber to be made rich without causing the production of smoke.